Updating sub-themes

I was wondering this too - having just come back to a rather dormant project (as yet offline) and decided (perhaps rashly :-) ) that I'd recreate it in D7 rather than continue in D6!

I found this page:
http://drupal.org/update/themes/6/7, "Converting 6.x themes to 7.x"

My initial impression is that there isn't an easy way but it'll be a step by step process, looking for differences in how things work and reviewing/recreating/amending files one by one. This is because there are lots of changes in core - like things being renamed or just working in a completely different way - so there would be things in the old subthemes which wouldn't make sense any more.

I think for Zen the process will be something like:

Install Zen D7. Proceed as if you'd never done a Zen subtheme before, and copy, rename & set up the Zen-for-D7 STARTERKIT directory, following its README.txt, to create a new subtheme.

Note which files you made custom versions of for your subtheme in D6. Copy the STARTERKIT-origin files with the same names/functions.

Compare the Zen-D6 files you'd customised before with the Zen-D7-STARTERKIT-copy ones. Do equivalent customisations.

But like I say, this is just a first impression from a couple of hours' reading and tinkering. Comments from more experienced Drupallers would be very welcome.

Actually, Jennifer, I think you've done a wonderful job of outlining the steps necessary to port a theme to Drupal 7. :-)

::grins::
how nice of you to say :-)

Funnily enough I was just on my way back to this thread today, to add a couple of other useful resources to it (partly for my own benefit):
Zen's stylesheets - http://drupal.org/node/1069786
How to build your own sub-theme (7.x-3.x) - http://drupal.org/node/1010576